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B737 flight crew is curious why DME deferral prohibits aircraft from RNAV approaches and Q routes but allows LNAV use for airways navigation.
| Attributes | |
| ACN | 785145 |
| Time | |
| Date | 200805 |
| Local Time Of Day | 0601 To 1200 |
| Place | |
| Locale Reference | airport : zzz.airport |
| State Reference | US |
| Altitude | agl single value : 0 |
| Environment | |
| Light | Dawn |
| Aircraft 1 | |
| Operator | common carrier : air carrier |
| Make Model Name | B737-300 |
| Operating Under FAR Part | Part 121 |
| Flight Phase | ground : preflight |
| Flight Plan | IFR |
| Person 1 | |
| Affiliation | company : air carrier |
| Function | flight crew : captain oversight : pic |
| Experience | flight time last 90 days : 250 flight time total : 10000 flight time type : 3500 |
| ASRS Report | 785145 |
| Events | |
| Anomaly | aircraft equipment problem : less severe |
| Independent Detector | other flight crewa |
| Resolutory Action | none taken : anomaly accepted |
| Supplementary | |
| Problem Areas | Aircraft |
| Primary Problem | Aircraft |
Narrative:
Aircraft had first officer DME inoperative deferral. The restr was do not RNAV departures; RNAV rtes; RNAV arrs; or Q rtes. The flight was filed as B733/west. I called dispatch and got a maintenance 3-WAY patch to discuss the restrs and to inquire as to if LNAV was still available for use since the restr did not forbid its use. On the B737 the navigation computers only require 1 operable DME to function and I was curious about not being able to use RNAV rtes but not being restr from using LNAV for jet rtes and non RNAV departures/arrs. Maintenance said LNAV was indeed available for use and could not explain the reason for the restr either; since we use LNAV to fly the restr profiles also. Maintenance cleared us to fly back on the jet route filed flight plan using LNAV but suggested we file a safety report to have the issue looked into. We flew the route filed and did not accept any direct down route clrncs; due to being a /west; and filed this report as suggested. An answer as to why we were restr from RNAV routing but not LNAV when the same system controls both would be appreciated.
Original NASA ASRS Text
Title: B737 FLT CREW IS CURIOUS WHY DME DEFERRAL PROHIBITS ACFT FROM RNAV APPROACHES AND Q ROUTES BUT ALLOWS LNAV USE FOR AIRWAYS NAVIGATION.
Narrative: ACFT HAD FO DME INOP DEFERRAL. THE RESTR WAS DO NOT RNAV DEPS; RNAV RTES; RNAV ARRS; OR Q RTES. THE FLT WAS FILED AS B733/W. I CALLED DISPATCH AND GOT A MAINT 3-WAY PATCH TO DISCUSS THE RESTRS AND TO INQUIRE AS TO IF LNAV WAS STILL AVAILABLE FOR USE SINCE THE RESTR DID NOT FORBID ITS USE. ON THE B737 THE NAV COMPUTERS ONLY REQUIRE 1 OPERABLE DME TO FUNCTION AND I WAS CURIOUS ABOUT NOT BEING ABLE TO USE RNAV RTES BUT NOT BEING RESTR FROM USING LNAV FOR JET RTES AND NON RNAV DEPS/ARRS. MAINT SAID LNAV WAS INDEED AVAILABLE FOR USE AND COULD NOT EXPLAIN THE REASON FOR THE RESTR EITHER; SINCE WE USE LNAV TO FLY THE RESTR PROFILES ALSO. MAINT CLRED US TO FLY BACK ON THE JET RTE FILED FLT PLAN USING LNAV BUT SUGGESTED WE FILE A SAFETY RPT TO HAVE THE ISSUE LOOKED INTO. WE FLEW THE RTE FILED AND DID NOT ACCEPT ANY DIRECT DOWN RTE CLRNCS; DUE TO BEING A /W; AND FILED THIS RPT AS SUGGESTED. AN ANSWER AS TO WHY WE WERE RESTR FROM RNAV ROUTING BUT NOT LNAV WHEN THE SAME SYS CTLS BOTH WOULD BE APPRECIATED.
Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.